This consideration will help us to realize that all reality which is produced by change, and subject to change, is essentially a mixture of _becoming_ and _being_, of _potential_ and _actual_. The reality of such being is not _tota simul_. Only immutable being, whose duration is _eternal_, has its reality _tota simul_: it alone is _purely actual_, the “_Actus Purus_”; and its duration is one eternal “_now_,” without beginning, end, or succession. But mutable being, whose duration in actual existence is measured by _time_, is actualized only successively: its actuality at any particular instant does not embody the whole of its reality: this latter includes also a “_was_” and “_will be_”; the thing was _potentially_ what it now is _actually_, and it will become actually something which it now is only potentially; nor shall we have understood even moderately the nature or essence of any mutable being—an oak-tree, for example—until we have grasped the fact that the whole reality of its nature embraces more than what we find of it actually existing at any given instant of its existence. In other words, we have to bear in mind that the reality of such a being is not pure actuality but a mixture of potential and actual: that it is an _actus non-purus_, or an _actus mixtus_. We have to note well that the _potential being_ of a thing is something _real_—that it is not merely a _modus loquendi_, or a _modus intelligendi_. The oak is in the acorn in some true and real sense: the potentiality of the oak is something real in the acorn: if it were not so, if it were nothing real in the acorn, we could say with equal truth that a man or a horse or a house is potentially in the acorn; or, again with equal truth, that the oak is potentially in a mustard-seed, or a grain of corn, or a pebble, or a drop of water. Therefore the oak is _really_ in the acorn—not actually but potentially, _potentia passiva_. The oak-tree is also really in those _active_ forces of nature whose influence on the acorn develop the latter into an actual oak-tree: it is in those causes not actually, of course, but _virtually_, for they possess in themselves the _operative power_—_potentia activa sive operativa_—to educe the oak-tree out of the acorn. These two potential conditions of a being—in the active causes which produce it, and in the pre-existing actual thing or things from which it is produced—are called each a real or subjective potency, _potentia realis_, or _potentia subjectiva_, in distinction from the mere logical or objective possibility of such a being.